Effect of Substrate Contact Shape and Placement on RF Characteristics of 45 nm Low Power CMOS Devices

The substrate resistance of 45 nm CMOS devices shows a strong dependence on the distance between the device edge and the substrate contact ring, as well as on the number of sides that the surrounding ring contacts the substrate. We find that the unilateral gain is impacted by the substrate resistance (R_sx) through the gate-body capacitance feedback path at low to medium frequencies (< 20 GHz). At mm-wave frequencies, the unilateral power gain is affected by R_sx through the drain-body capacitance pole, and the unilateral power gain deviates from the ideal -20 dB/dec slope. Within the range of designs that have been studied, the impact of substrate resistance on f_T, maximum available gain, high frequency noise and power characteristics of the devices is minimal.